U.S. Pat. No. 6,068,529 discloses an improved twin propeller marine propulsion unit. A vertical drive shaft is journaled in the lower gearcase and drives a pair of bevel gears. A pair of concentric propeller shafts is mounted in the lower torpedo section of the gearcase and each shaft carries a propeller. A slidable clutch is movable between a neutral, a forward, and a reverse position and serves to operably connect the outer propeller shaft with one of the bevel gears when the clutch is moved to the forward drive position. A gear is mounted for sliding movement in unison with the clutch and acts to operably engage the inner propeller shaft with the second bevel gear when the clutch is in the forward drive position so that both propellers are driven in opposite directions to provide forward motion for the watercraft. The propulsion unit also includes a dual cooling water pick-up system in which seawater is drawn to the water pump both through a series of vertical inlet ports in the gearcase and through a plurality of inlet holes that are located in the forward end of the lower torpedo section. Exhaust gas from the engine is discharged through the rear end of the lower housing section through axial passages in the hub of the forward propeller and then across the outer surface of the rear propeller.
U.S. Pat. No. 5,816,869 discloses a propeller for a marine propulsion system that provides variable length exhaust paths depending upon the speed of the boat and motor. The propeller includes a propeller hub and an exhaust tube positioned within the propeller hub. The exhaust tube extends past the aft end of the propeller hub and defines a first exhaust passageway. A second exhaust passageway is positioned between the propeller hub and the exhaust tube. The second exhaust passageway is shorter than the first exhaust passageway. At low speeds, engine exhaust exits the longer first passageway, while at moderate speeds, engine exhaust exits the shorter second passageway. Therefore, the effective length of the exhaust path varies depending upon the speed of the motor, such that the length of the exhaust path is specifically tuned to several speeds of the motor
U.S. Pat. No. 4,871,334 discloses a marine propulsion device includes a drive housing to which is attached a suitable engine, the exhaust of which is pumped downwardly through a suitable passage in the drive housing to adjacent a torpedo housing carrying at least one propeller. A generally horizontal anti-ventilation plate is disposed above the torpedo housing, and a strut extends between the plate and the torpedo housing, just forwardly of the upper portion of the propeller. Substantially all of the engine exhaust is forced by the engine from the drive housing passage for discharge into the path of the upper portion of the propeller. In one embodiment, substantially all of the exhaust passes through the strut and is discharged rearwardly therefrom into ventilating engagement with the forward face of the propeller. In another embodiment, a portion of the exhaust is also discharged downwardly through the anti-ventilation plate onto the upper edge portion of the propeller.
U.S. Pat. No. 4,642,057 discloses a marine propeller mounting arrangement that includes a sleeve member for mounting on a propeller shaft, a propeller having an inner hub which fits over the sleeve member and a cushion member fitting between the sleeve member and the propeller inner hub. The sleeve member includes radially extending projections registering with channels in the hub to positively drive the propeller, even in the event of failure of the cushion member. The propeller has an outer hub surrounding the inner hub to define an exhaust gas passageway through the propeller.